1. Reminder About the Principle of OFDM Modulation
OFDM modulation consists of making a distribution in the time-frequency space of symbols with duration Tu (called the useful symbol time) on a plurality of carrier frequencies that are independently modulated for example in QPSK (Quadrature Phase Shift Keying) or QAM (Quadrature Amplitude Modulation), for example with 16, 64, 256, etc. states. The OFDM technique thus breaks the channel into cells along the time axis 11 and the frequency axis 12 as illustrated in FIG. 1. Each carrier is orthogonal to the previous carrier.
The channel with predetermined width 13 is decomposed into a sequence of frequency sub-bands 14 and a sequence of time segments 15 (also called time intervals).
A dedicated carrier is assigned to each time/frequency cell. Therefore, we will distribute the information to be transported over all these carriers, each modulated at low speed, for example by a QPSK or QAM type modulation. An OFDM symbol comprises all information carried by the set of carriers at time t.
This modulation technique is particularly efficient in situations in which multipaths are encountered. Thus, as illustrated on FIG. 2 that presents a set of OFDM symbols 21, the same sequence of symbols reaching a receiver by two different paths is like the same information arriving at two different instants and that is additive. These echoes cause two types of faults:                intra-symbol interference: addition of a symbol with itself with a slight phase shift,        inter-symbol interference: addition of a symbol with the next symbol and/or the previous symbol with a slight phase shift.        
A “dead” zone called a guard interval 22 is inserted between transmitted symbols, the duration Δ of which is chosen to be sufficiently large compared with spreading of echoes. These precautions will limit inter-symbol interference (as the interference is absorbed by the guard interval). Thus, each OFDM symbol 21 comprises a guard interval 22 and data 23.
On reception, carriers may also have been attenuated (destructive echoes) or amplified (constructive echoes) and/or subject to a phase rotation.
Pilot synchronisation carriers, also called reference pilots (often with an amplitude greater than the useful data carriers), are also inserted to calculate the channel transfer function so that the signal can be equalised before demodulation. The value and position of these reference pilots in time/frequency space are predefined and known to receivers.
FIG. 3 thus presents the OFDM structure in mode A of a set of DRM symbols, illustrating the distribution of reference pilots 31 in the time/frequency space. This structure is described particularly in DRM standard ETSI ES 201 980.
FIG. 4 presents another example of an OFDM structure of a set of DVB-T symbols illustrating the distribution of reference pilots 41 among useful data 42 in the time/frequency space.
The result obtained after interpolation in time and in frequency is a more or less relevant estimation of the channel response, depending on the number of reference pilots and their distribution in the time/frequency domain.
Thus, the reference pilots inserted in the multicarrier signal are used to estimate the propagation channel. The estimation of the propagation channel is used notably to correct received data, also called the data pilots at the receiver (equalisation) and to obtain the pulse response of the propagation channel. The pulse response obtained may then be used to refine the time synchronisation of the receiver(s).
2. Application in AM Bands (DRM)
OFDM modulation is increasingly used in digital broadcasting because it is very well adapted to variations of the radio channel that are related essentially to echoes and the Doppler effect. It was thus selected for digital audio broadcasting in the AM bands (DRM).
To choose the most suitable OFDM structure, engineers start by studying the characteristics of the radio channel that vary as a function of the emission frequency, of the passband of the signal and, for digital audio broadcasting in the AM bands (DRM), also of the propagation conditions during the day and the night and solar cycles.
Receivers used for OFDM demodulation essentially use the channel response calculated from reference pilots. Therefore the accuracy of this estimate depends on the proportion of reference pilots inserted in OFDM symbols.
3. Interfering (or Jamming) Signals
Parasite signals can be added to the useful signal during a transmission between a transmitter and a receiver, and can disturb reception of the useful signal if they exceed a given threshold.
This threshold depends particularly on the characteristics of the receiver and the channel on which jamming is received, compared with the useful signal channel. Thus, we talk about “co-channel jamming” when the interfering signals and the useful signal are transported on the same channel, and “adjacent channel jamming” when the interfering signals and the useful signal are transported on adjacent channels.
For example in the AM band, the useful signal may be disturbed by pulse type jamming caused by human activities (automobile, industrial and medical equipment, lighting fixtures, etc.) and/or by narrow band jamming related to other transmissions in these AM bands (AM radio, communication systems, radar, etc.).
Note that the presence of these other transmissions prevents an increase in the power of a DRM band that would otherwise disturb neighbouring bands and for example prevent good quality reception of classical AM analogue signals. Filters used on existing receivers are not sufficient to perfectly eliminate neighbouring bands.
Several techniques for elimination, or at least for the reduction of jamming are already known to those skilled in the art.
Thus, in patent application EP 1087579 (Canceller for jamming wave by interference) K. Shibuya et al. present a technique for cancellation of jamming created by interference in a system using an OFDM type modulation.
In patent CA 1186742 (Interference cancelling system for a mobile subscriber access communications system) Frank S. Gutleber et al. also propose a system for cancellation of interference in a conventional communications system that does not use an OFDM type modulation.
A technique for measuring jamming in radar was also proposed by Bernard L. Lewis et al. in U.S. Pat. No. 5,359,329.
Finally, M. Lanoiselee et al. present a technique for cancellation of jamming in patent FR 2 753 592 (“Procédé de démodulation de signaux numériques émis par salves robuste aux brouilleurs à bande étroite—Method of demodulating digital signals emitted in bursts, resistant to narrow band jammers”) based on frequency processing of jammers.
4. Disadvantages of Techniques According to Prior Art
These techniques for eliminating jammers are based on a priori knowledge of the jammers and/or the constancy of the useful signal, and therefore take place earlier than the channel estimating and/or received data equalising steps.
They usually require the identification of “peaks” located above the average of OFDM symbol pilots before the channel can be correctly estimated and data pilots can be equalised. Thus according to prior art, “peaks” located in the time domain are observed and are eliminated.
However, these solutions are complex and difficult to use for two main reasons:                firstly, OFDM symbol data pilots are at very different levels, due to modulation at several levels (MAQ16, MAQ64, etc.);        secondly, the channel generates very large amplitude and phase variations between pilots.        